Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Input devices based on touch sensing (touch screens) have long been used in electronic devices such as computers, personal digital assistants (PDAs), handheld games and point of sale kiosks, and are now appearing in other portable consumer electronics devices such as mobile phones. Generally, touch-enabled devices allow a user to interact with the device by touching one or more graphical elements, such as icons or keys of a virtual keyboard, presented on a display.
Several touch-sensing technologies are known, including resistive, surface capacitive, projected capacitive, surface acoustic wave, optical and infrared, all of which have advantages and disadvantages in areas such as cost, reliability, ease of viewing in bright light, ability to sense different types of touch object, e.g. finger, gloved finger, stylus, and single or multi-touch capability.
The various touch-sensing technologies known differ widely in their multi-touch capability, i.e. their performance when faced with two or more simultaneous touch events. Some early touch-sensing technologies such as resistive and surface capacitive are completely unsuited to detecting multiple touch events, reporting two simultaneous touch events as a ‘phantom touch’ halfway between the two actual points. Certain other touch-sensing technologies have good multi-touch capability but are disadvantageous in other respects. For example projected capacitive touch screens, discussed in US Patent Application Publication No 2006/0097991 A1, only sense certain touch objects (e.g. gloved fingers and non-conductive styluses are unsuitable) and use high refractive index transparent conductive films that are well known to reduce display viewability, particularly in bright sunlight. In another example video camera-based systems, discussed in US Patent Application Publication Nos 2006/0284874 A1 and 2008/0029691 A1, are extremely bulky and unsuitable for hand-held devices. Another touch technology with good multi-touch capability is ‘in-cell’ touch, where an array of sensors are integrated with the display pixels of a display (such as an LCD or OLED display). These sensors are usually photo-detectors (disclosed in U.S. Pat. No. 7,166,966 and US Patent Application Publication No 2006/0033016 A1 for example), but variations involving micro-switches (US 2006/0001651 A1) and variable capacitors (US 2008/0055267 A1), among others, are also known. In-cell approaches cannot be retro-fitted and generally add complexity to the manufacture and control of the displays in which the sensors are integrated. Furthermore those that rely on ambient light shadowing cannot function in low light conditions.
In yet another approach to touch sensing with several possible configurations, a touch event is detected by the shadowing of two paths in a sheet of light (usually in the infrared region) established in front of a display. In one such configuration, illustrated in FIG. 1 and described in U.S. Pat. Nos. 4,507,557, 6,943,779 and 7,015,894, a touch input device 1 includes a pair of optical units 2 in adjacent corners of a rectangular input area 4 and a retro-reflective layer 6 along three edges of the input area. Each optical unit includes a light source emitting a fan of light 8 across the input area, and a photo-detector array (e.g. a line camera) where each detector pixel receives light retro-reflected from a certain portion of the retro-reflective layer 6. A touch object 10 in the input area prevents retro-reflected light reaching one or more detector pixels in each photo-detector array, and its position is determined by triangulation. For the purposes of this specification, touch input devices that operate in this manner will be referred to as ‘optical’ touch input devices.
In another configuration, illustrated in FIG. 2 and described in U.S. Pat. Nos. 3,478,220 and 3,764,813, a touch input device 11 includes arrays of discrete light sources 12 (e.g. LEDs) along two adjacent sides of a rectangular input area 4 emitting two sets of parallel beams of light 16 towards opposing arrays of photo-detectors 14 along the other two sides of the input area. If a touch object 10 in the input area blocks a substantial portion of at least one beam in each of the two axes, its location can be readily determined.
In a variant touch input device 17 that greatly reduces the optoelectronic component count, illustrated in FIG. 3 and described in U.S. Pat. No. 5,914,709, the arrays of light sources are replaced by arrays of ‘transmit’ optical waveguides 18 integrated on an L-shaped substrate 20 that distribute light from a single light source 12 via a 1×N splitter 21 to produce a grid of light beams 16, and the arrays of photo-detectors are replaced by arrays of ‘receive’ optical waveguides 22 integrated on another L-shaped substrate 23 that collect the light beams and conduct them to a detector array 24 (e.g. a line camera or a digital camera chip). Each optical waveguide includes an in-plane lens 26 that collimates the signal light in the plane of the input area 4, and the input device may also include cylindrically curved vertical collimating lenses (VCLs) 28 to collimate the signal light in the out-of-plane direction. As in the input device 11 of FIG. 2, a touch object is located from the beams blocked in each axis. For simplicity, FIG. 3 only shows four waveguides per side of the input area 4; in actual touch input devices the in-plane lenses will be sufficiently closely spaced such that the smallest likely touch object will block a substantial portion of at least one beam in each axis.
In yet another variant touch input device 30 shown in FIG. 4 and disclosed in US Patent Application Publication No 2008/0278460 A1, entitled ‘A transmissive body’ and incorporated herein by reference, the ‘transmit’ waveguides 18 and their in-plane lenses 26 of the device 17 shown in FIG. 3 are replaced by a transmissive body 32 including a planar transmissive element 34 and two collimation/redirection elements 36 that include parabolic reflectors 38. Infrared light 40 from a pair of optical sources 12 is launched into the transmissive element, then collimated and re-directed by the collimation/redirection elements to produce two sheets of light 42 that propagate in front of the transmissive element towards the receive waveguides 22, so that a touch object can be detected and its dimensions determined from those portions of the light sheets 42 blocked by the touch object. Clearly the transmissive element 34 needs to be transparent to the infrared light 40 emitted by the optical sources 12, and it also needs to be transparent to visible light if there is an underlying display (not shown). Alternatively, a display may be located between the transmissive element and the light sheets 42, in which case the transmissive element need not be transparent to visible light.
A common feature of the touch input devices shown in FIGS. 2, 3 and 4 is that the sensing light is provided in two fields containing parallel rays of light, either as discrete beams (FIGS. 2 and 3) or as more or less uniform sheets of light (FIG. 4). The axes of the two light fields are usually perpendicular to each other and to the sides of the input area, although this is not essential (see for example U.S. Pat. No. 5,414,413). For the purposes of this specification, touch input devices that operate in this manner will be referred to as ‘infrared’ touch input devices. However it should be understood that the wavelength of the sensing light need not be in the infrared region, but could be in the visible for example.
In all of the ‘optical’ and ‘infrared’ touch input devices shown in FIGS. 1 to 4, a touch event is detected by the shadowing of two light paths. Turning now to the issue of multi-touch capability, although these touch input devices can detect the presence of multiple touch events, they are often unable to determine their locations unambiguously. In general, n simultaneous touch events will be detected as n2 ‘candidate points’, of which n(n−1) will be ‘phantom points’. For the simplest multi-touch situation n=2 (‘double touch’), the responses of an ‘optical’ touch input device 1 and an ‘infrared’ touch input device 11 are illustrated in FIGS. 5 and 6 respectively; in each case the ‘candidate points’ include the two actual touch points 10 and two ‘phantom points’ 44 at the corners of a quadrilateral, and it can be difficult if not impossible to identify the correct pair without further information. It will be appreciated that the variant ‘infrared’ touch input devices of FIGS. 3 and 4 will also respond in the manner shown in FIG. 6. In some circumstances the correct pair can be identified via some form of extra information: for example as explained in U.S. Pat. No. 6,856,259, touch-down and lift-off timing, relative object sizes and expected touch locations can all be of use in resolving the ambiguity.
However even if the correct pair can be identified, say because one touch-down occurred before the other, further complications can arise if the detection system has to track moving touch objects. For example if two moving touch objects (FIG. 7A) on an ‘infrared’ touch input device 11 move into an ‘eclipse’ state (FIG. 7B), the ambiguity between the actual points 10 and the phantom points 44 recurs when the objects move out of the eclipse state. FIGS. 7C and 7D illustrate two possible motions out of the eclipse state shown in FIG. 7B that in general are indistinguishable to the controller of the touch input device. It will be appreciated that a similar problem occurs with ‘optical’ touch input devices when two moving touch objects move into an ‘eclipse’ state.
For ‘optical’ touch input devices, various modifications are known that improve their multi-touch capability. Referring to FIG. 5, in one modification disclosed in US Patent Application Publication No 2006/0232830 A1, the two optical units 2 are replaced by binocular units each containing two slightly offset combinations of an emitter and a photo-detector array. If a double touch ambiguity is detected using one emitter/detector combination in each corner, the other combinations are activated. When comparing the candidate points obtained from the different combinations, the actual touch points 10 remain fixed while the ‘phantom points’ 44 appear to move, and can therefore be eliminated. US 2006/0232830 A1 also discusses another modification where a third optical unit, this time containing an emitter and a photo-detector array with a 180° field of view, is located in the middle of the input area edge between the two optical units 2. This third optical unit also makes use of the retro-reflective layer 6. It will be appreciated that because ‘infrared’ touch input devices have a very different configuration where the emitters and detectors are spaced apart across the input area, these known solutions for improving the multi-touch capability of ‘optical’ touch input devices are not necessarily applicable to ‘infrared’ touch input devices.